The open panel now occupies layout space: #root (the AppFrame grid) gives
up the panel width on the right via margin-right driven by a
--dsh-sidebar-width CSS variable, and only the center column is flexible,
so the squeeze lands exactly on the agent output and the input bar (VSCode
sidebar behavior). The fixed panel covers the vacated strip; expand/
collapse animates the margin and the slide on the same theme duration,
width drags disable the transition, and prefers-reduced-motion is honored.